In a centrifugal impeller shown in FIGS. 1A and 1B, an inlet width B1 and an outlet width B2 of a blade 110, an inlet diameter D0 and an outlet diameter D2 of the centrifugal impeller, and an inlet angle β1 and an outlet angle β2 of the blade 110 are designed so as to satisfy a required flow rate and a required pump head. In the conventional centrifugal impeller, it is desirable to change the width of the blade 110 gradually from the inlet width B1 to the outlet width B2, and it is also desirable to change the angle of the blade 110 gradually from the inlet angle β1 to the outlet angle β2.
FIGS. 2A and 2B are meridional-plane cross-sectional views showing a conventional centrifugal impeller designed as stated above. As shown in FIGS. 2A and 2B, the centrifugal impeller has a plurality of blades 110 disposed between a shroud 120 and a hub 130 (only one blade is shown in FIGS. 2A and 2B). The blades 110 are arranged at angularly equal intervals in a circumferential direction of the centrifugal impeller. A fluid path 140 is formed by adjacent two of the blades 110, the shroud 120, and the hub 130 so that a fluid flows through the fluid path 140. In the conventional centrifugal impeller shown in FIG. 2A, the shroud 120 curves entirely so as to project toward the hub 130 to form a curved line L1. In the conventional centrifugal impeller shown in FIG. 2B, the shroud 120 is inclined straightly toward the hub 130 to form a straight line L2.
However, as shown in FIGS. 2A and 2B, if the curved line L1 or the straight line L2 is formed at the shroud 120, a meridional length of the fluid path 140 becomes long and a width of the whole fluid path 140 in the meridional-plane cross-section becomes small in the case of the centrifugal impeller of a small flow rate and a high pump head, i.e. a small specific speed (Ns). Consequently, a relative velocity of the fluid flowing through the fluid path 140 becomes large, and hence a friction loss in the fluid path 140 is increased, thus lowering an impeller performance.